Technical Field
The invention relates to the art of disc brakes for heavy-duty vehicles. More particularly, the invention relates to rotors of disc brakes for heavy-duty vehicles. Still more particularly, the invention is directed to a rotor of a disc brake, referred to herein as a brake rotor, which includes a reduced diameter for use in conjunction with reduced-diameter wheel end configurations, and which includes features that reduce the peak temperature of the rotor, reduce coning of the rotor during braking, and desirably maintain the heat transfer properties of the rotor while decreasing the brittleness and improving the hot strength of the rotor.
Background Art
Disc brake systems for vehicles are well known in the brake art. Such systems operate by forcing a pair of opposing brake pads against a rotor, thereby creating friction between the pads and the rotor to enable slowing and/or stopping of the vehicle. More particularly, a disc brake system includes a plurality of disc brake assemblies, in which each assembly is operatively mounted on or adjacent a wheel end of the vehicle.
Each disc brake assembly includes a carrier, which supports a caliper that is described in greater detail below, and is attached to a torque plate, typically by mechanical fasteners, such as bolts. The torque plate in turn is rigidly connected to an axle of an axle/suspension system of the vehicle, such as by welding. The torque plate resists the torque that is generated during braking, and maintains proper alignment of the carrier and caliper to ensure optimum operation of the components of the brake assembly.
As mentioned above, the carrier supports a caliper, and the caliper is formed with a bore for receiving one or more pistons and an actuator. The actuator typically is a brake air chamber, which is in fluid communication with a compressed air source and activates movement of the piston(s). The caliper also includes a reaction arm that is disposed opposite the piston(s). Each one of a pair of opposing brake pads includes friction material that is mounted on a backing plate, and is seated in the carrier, with one of the pads being adjacent the piston(s) and the other pad being adjacent the reaction arm. Upon actuation by the actuator, the piston(s) and the reaction arm cooperate to control movement of the brake pads.
The rotor includes a disc portion, which is disposed between the brake pads in a manner that allows the friction material of each pad to face a respective surface of the disc portion. The rotor also includes a mounting portion that is adapted for mounting to a respective wheel end assembly of the vehicle by mechanical fasteners, such as bolts. A sleeve typically is integrally formed with and extends between the disc portion and the mounting portion of the rotor. This construction enables the rotor to be rigidly connected to the wheel end assembly, and thus to a respective vehicle wheel.
In the prior art, many heavy-duty vehicles have employed a wheel that includes a diameter of 22.5 inches (571.5 millimeters). When an air disc brake system is employed in conjunction with such 22.5-inch wheels, the brake rotor diameter typically is about 16.9 inches (430 mm). In order to lower the floor height of a heavy-duty vehicle to increase the vertical space available for cargo, certain heavy-duty vehicle designs have moved from a 22.5 inch diameter wheel to a wheel with a reduced diameter. For example, such reduced-diameter wheel designs have employed a 17.5 inch (444.5 mm) diameter wheel, rather than a 22.5 inch diameter wheel.
In the prior art, it was common to employ a drum brake system with a 17.5 inch diameter wheel. Due to the improved performance characteristics of an air disc brake system when compared to a drum brake system, it is desirable to enable the use of an air disc brake system with a reduced-diameter 17.5 inch wheel, rather than a drum brake system. However, the use of an air disc brake system with a 17.5 inch wheel poses certain challenges.
For example, it is desirable for heavy-duty vehicles, including trailers that are classified as National Highway Traffic Safety Administration (NHTSA) Class 8 trailers, to maintain certain brake certifications, such as the Federal Motor Vehicle Safety Standards (FMVSS)-121 brake certifications for a 20,000 pound gross axle weight rating (GAWR). Such a rating is typical for air disc brakes that are employed with larger 22.5 inch diameter wheels, but poses a challenge for air disc brakes that are employed with reduced 17.5 inch diameter wheels.
More particularly, air disc brakes, including brake rotors, which are employed with a 17.5 inch diameter wheel are limited in their outer diameter, as they must be packaged so that their outer diameter is well inside of the 17.5 inch wheel diameter. Such air disc brakes are also limited in their inner diameter, as they must still clear the outer diameter of the axle, which is maintained at about a 5-inch outside diameter to adequately support the 20,000 pound GAWR. It is also desirable for an air disc brake system on such a heavy-duty vehicle to provide acceptable performance and life of the brake rotor and the brake pads.
Traditionally, prior art rotors in such a small air disc brake package, which are referred to herein for the purpose of convenience as reduced-diameter brake rotors, have been ventilated rotors, which are rotors that include two rotor discs with vanes or pins connecting the discs to one another. Because of the above-described outer diameter and inner diameter limitations, such ventilated rotors have had durability issues. More particularly, the frictional forces generated from braking impart high temperature cycles on the brake rotor. The highest temperature actually experienced by the rotor, which is known in the art as the peak temperature of the rotor, depends on the specific construction and features of each rotor. In the prior art, reduced-diameter ventilated rotors have experienced high peak temperatures that have caused them to undesirably wear out rapidly, and to experience thermal cracking of the rotor faces in demanding applications. Therefore, it is desirable to provide a reduced-diameter brake rotor with a construction that reduces the peak temperature of the rotor.
In addition, the mechanical connection between the brake rotor and the wheel hub enables heat transfer from the rotor to the wheel hub. When the rotor experiences high temperatures, such heat transfer in turn creates high temperatures in the wheel hub. High temperatures in the wheel hub can prematurely age the wheel seals and the lubricant in the hub, thereby undesirably shortening the service interval on the wheel end assembly. As a result, it is again desirable to provide a reduced-diameter brake rotor with a construction that reduces the peak temperature of the rotor, which reduces the heat that is transferred to the wheel hub and desirably extends the life of the hub seals and lubricant.
Moreover, for optimum functioning of the brake system, it is desirable for the disc portion of the rotor to be maintained in a vertical orientation, which provides a square, even contact of the inboard surface of the disc with the friction material of its adjacent brake pad, and a square, even contact of the outboard surface of the disc with the friction material of its adjacent brake pad. However, it is known in the art that the heat which is generated from the friction of the brake pads being forced against the disc portion of the rotor causes the disc portion to expand radially. In addition, the heat generated from the friction of the brake pads being forced against the disc portion of the rotor creates a thermal gradient across the rotor sleeve, with the sleeve being hotter in the inboard area that is adjacent the rotor disc portion than the outboard area that is away from the rotor disc portion. This thermal gradient causes the rotor sleeve to expand at the inboard area that is adjacent the rotor disc portion. The radial expansion of the disc portion of the rotor and the expansion of the rotor sleeve adjacent the rotor disc portion causes the outer perimeter of the disc portion to move slightly from its desired vertical orientation and in the direction of the rotor sleeve. The rotor disc portion thereby angles or tilts in the direction of the rotor sleeve, that is, in the outboard direction. Such tilting of the rotor disc portion due to the heat generated from braking is referred to in the art as coning.
When the rotor disc experiences coning, the inboard surface of the disc is no longer in square, even contact with the friction material of its adjacent brake pad, and the outboard surface of the disc is no longer in square, even contact with the friction material of its adjacent brake pad. Such uneven contact between the inboard surface and its adjacent brake pad, and the outboard surface and its adjacent brake pad, reduces the efficiency of the brake system. In addition, such uneven contact may create stress areas at each point of uneven contact between the brake pads and the inboard and outboard disc surfaces, which may cause the formation of cracks in the rotor disc, thereby reducing the life of the rotor. Such uneven contact between the inboard disc surface and its adjacent brake pad, and the outboard disc surface and its adjacent brake pad, also undesirably reduces the life of the brake pads. As a result, it is desirable to provide a reduced-diameter brake rotor construction that reduces potential coning, and in turn increases the life of the brake pads and the rotor.
Also, rotors for heavy-duty vehicles have traditionally been formed from cast iron in order to exhibit the strength, hardness and stability required for the braking operation. In the prior art, such rotors have been formed with a high carbon content to maintain the heat transfer properties of the rotor disc, which enables the rotor to dissipate heat to reduce the thermal stresses on the rotor, thereby improving the performance and life of the rotor. For example, in the prior art, the carbon content of a rotor has typically been greater than about four (4) weight percent (%) carbon. While such a high carbon content has provided good heat transfer properties, it creates a rotor that is undesirably brittle. When a rotor is brittle, it has little ability to resist high temperature stresses, and as a result, prior art rotors with a high carbon content may undesirably experience crack initiation and propagation. In addition, such a high carbon content undesirably reduces the strength of the rotor at elevated temperatures, which is also referred to in the art as a reduction of the rotor's hot strength. Therefore, it is desirable to provide a reduced-diameter brake rotor that includes a metallurgical composition which desirably maintains the heat transfer properties of the rotor, while decreasing the brittleness and improving the hot strength associated with prior art high-carbon compositions.
As a result, there is a need in the art for a reduced-diameter brake rotor for heavy-duty vehicles that provides a construction that reduces the peak temperature of the rotor, reduces coning of the rotor during braking, and includes a metallurgical composition which desirably maintains the heat transfer properties of the rotor, while decreasing the brittleness and improving the hot strength of the rotor. The reduced-diameter brake rotor for heavy-duty vehicles of the present invention satisfies these needs, as will be described in detail below.